The present invention relates to a formed member made of a steel sheet member, for example, a panel member or structural member for a body of vehicles such as automobiles, and a method for producing the same.
As is commonly known, in recent years, in the field of vehicles such as automobiles and the like, there is a strong request for improving fuel efficiency from the view point of environmental problems, in addition to a request for improving the cost efficiency in vehicle running. In particular, it is requested to further reduce the weight of a body which forms a major part of the vehicle weight. On the other hand, in order to keep and improve the passenger protecting performance in the case of a collision of the vehicle, it is necessary to ensure the body strength and the body rigidity of more than certain levels.
As described above, for a body of automobile, it is required to concurrently achieve ensuring of the body strength and the body rigidity for improving the safety in the case of a collision, and reducing of weight for improving the fuel efficiency.
As is well known in the art, in order to improve the passenger protecting performance in the case of a collision of the vehicle, it is necessary to increase the strength of the steel sheet itself used for forming a panel member and a structural member of the body, or improve the structural strength by increasing the thickness of the steel sheet or by providing an additional reinforcing member.
However, of these approaches, the latter (strengthening the structure) is difficult to be adopted in principle because increase of the thickness of the steel sheet and additional provision of a reinforcing member necessarily lead to increase of the body weight.
On other hand, as for the former approach (strengthening of the steel sheet itself), it is known that only steel sheets of classes having a tensile strength up to 440 MPa (so-called 440 MPa steel sheet, of which minimum value of the tensile strength in specification is 440 MPa, actually having a tensile strength in the range of about 440 to 550 MPa) can be applied in usual, because of the restriction in ensuring plastic formability of the steel sheet by press forming and the like, and that steel sheets of higher strength are extremely difficult to be formed.
In the case of the above range of the tensile strength (approximately 500 MPa or less), however, even if an attempt is made to decrease the thickness by using a steel sheet of high strength, it is difficult to achieve a sufficient effect in regard to reduction of body weight and reduction of production costs.
By the way, as one type of heat treatment for steel materials, it is conventionally known that by subjecting a steel material containing a nitriding element (so-called nitriding steel) to a certain heat treatment (nitriding treatment), it is possible to obtain a steel member with soft inside having extremely high hardness in the surface part and the vicinity.
This nitriding treatment essentially aims at improving the abrasion resistance of the surface part while restricting increase of the inside hardness of the member and ensuring its toughness, and does not intend to improve the strength of the whole member. However, in recent years, from the view of ensuring concurrently both reduced weight and strength/rigidity of the vehicle body, consideration is made to apply surface heat treatment techniques including such a nitriding treatment on structural members and reinforcing members for an automobile. For example, Japanese Unexamined Patent Laid-Open Publication HEI 11-279685 discloses that for the purpose of producing a high-strength press formed member having excellent impact absorptivity with satisfactory dimensional accuracy and a low production cost, a steel sheet of a specific composition is subjected to a nitriding treatment after press work is completed.
Thus, in the above prior art, an effort is made to improve the strength of a steel sheet itself while ensuring press formability by subjecting the nitriding steel sheet to a nitriding treatment after being press formed. In practice, however, the strength (tensile strength) obtained after the nitriding treatment is around 700 MPa maximum which is not satisfactory. Therefore, it is not possible to obtain a significant reduction of weight and production cost solely using the teachings of Japanese Unexamined Patent Laid-Open Publication HEI 11-279685.
To improve the performance of the vehicle in protecting passengers during a collision, the structural strength of the vehicle may be improved by increasing the thickness of the steel sheet used to form body panel member and structural member of the vehicle. However, this method of increasing structural strength by increasing the sheet thickness of such body parts and structural members is not preferred if such method does not significantly increase the rigidity of the whole body since vehicle body weight and the production cost is increased.
Therefore, it is desirable to provide a one-piece press formed member having portions with different properties from other portions of the formed member which may be, for example, a body panel member or a structural member.
With regards to such a request, a so-called tailored blank method is known as a method for forming a part having different properties from other parts in a one-piece member. The tailored blank method is to join, by welding and the like, blank materials (sheet-like members) having different properties for a predetermined specification (such as sheet thickness and/or heat treatment properties) to obtain one-piece sheet member (preform), and thereafter, to perform plastic forming such as press forming to obtain a final formed member.
By employing such a method, it is possible to obtain a formed member having a part therein of which properties is different from other parts for a predetermined specification in accordance with the properties of the blank material, while generally improving yield of the sheet-like material.
Therefore, by applying the tailored blank method to the production of bodies of automobiles, it is possible to improve yield of the steel sheet material, and to form a part having different properties from that of other parts in one-piece member constitutes a portion of vehicle body. Thereby, it is also possible to determine optimum material for the steel sheet material, sheet thickness, heat treatment, surface treatment and the like, in consideration of properties to be possessed (for example, strength, rigidity, corrosion resistance and the like) and the effects of reducing the body weight and reducing the production cost.
However, in the past, in a case where a formed member having a part of higher strength than that of the other parts is attempt to formed by applying the tailored blank method, the application of the method is limited to a certain range since only the steel sheet of which tensile strength is up to about 440 MPa can be utilized in view point of keep the press formability of the steel sheet as explained above. Therefore, it is hard to achieve sufficient effect in reducing the body weight and reducing the production cost.
In the tailored blank method, a plastic forming is conducted after welding the blank materials. Therefore, in particular, in a case where a blank material having high tensile strength is used, since the mechanical properties of the blank material is remarkably different from those of the other blank material, defects such as crack or break are apt to occur at the jointed portion. This may be a major factor to limit the application of the method.
By the way, as one of the methods for reinforcing a formed member, for example, a flame member used to a body of vehicles such as automobiles, it is known to fill foam material (foam urethane resin, for example) as filler material at least a part of inside the closed section of a formed member. Thereby, it is possible to improve the strength, the rigidity and the energy absorbing property for impact load, without large increases of weight as such cases where the sheet thickness of the formed member is increased, or reinforcements are provided.
In the case of improving the strength, the rigidity and the energy absorbing property of the formed member by filling filler material inside the closed section thereof, it is known that the reinforcing effect fluctuates not only depending on the type of filler material, the mechanical properties thereof and how to fill the filler material inside the section of the formed member, but also depending on the configuration to retain and/or fix the filler material inside the section of the formed member. For example, in the case where the shear adhesive strength of the fixing portion of the filling material to inside the formed member is a certain level or less, durability of the formed member of which a part is reinforced by filling material is reduced remarkably.
Accordingly, in the past, it was considered as the fixing method, for example, to apply adhesive bonds to an inner surface of a sheet member constitutes a formed member and fix the filler material to the formed member via the adhesive layer. However, this method is disadvantageous to improve the productivity of the reinforced formed member, since an additional step for applying the adhesive bond is required, and a long time is needed for drying the adhesive applied on the sheet material of the formed member.
On the other hand, it is considerable to employ a predetermined foaming filler material as the filler material and to fix the filler material onto the inner surface of the formed member by the adhesive force of the foaming filler material itself. However, it is hard to achieve a required value of the adhesive force stably.
By the way, a so-called hydroform process is commonly known as a forming process for forming a metal member. The process comprises the following steps: forming a preform having a shape relatively approximate to the final shape of the formed member by forming a metal member having a closed section; setting the preform in a predetermine mold; and forming the preform to the final shape corresponding to a shape of the mold by supplying the closed section space thereof with a pressurized fluid to apply an internal pressure inside the closed section of the preform.
By employing the hydroform process, it is possible to form integrally a formed member of relatively complex shape without assembling a plurality of members and joining them together. Thereby, it is possible to reduce the production cost by reducing the number of parts and eliminating working steps.
In the hydroform process, a pipe-like member such as a steel pipe formed in one-piece by drawing can be employed as a metal member to be subject to the process. Also, in the case where the thickness is relatively thin in comparison with the diameter, a pipe-like formed member obtained by rolling a steel sheet and welding the rolled steel member may be used as the metal member to be subject to the hydroform process.
However, in the past, in a case where a formed steel member of high strength is attempt to formed by applying the hydroform process, the application of the process is limited to a certain range since only the steel sheet of which tensile strength is up to about 440 MPa can be utilized in view point of keep the press formability of the steel sheet as explained above. Therefore, it is hard to achieve sufficient effect in reducing the weight of the formed member and reducing the production cost.
Furthermore, with regard to the formed member obtained by the hydroform process, it is not uncommon to be required to strengthen only a specified region. Therefore, responding such a request, it is desired to make it possible to perform a partial strengthening reliably and readily.
In view of the above, it is an essential object of the invention to securely obtain a formed member having sufficiently high strength after nitriding treatment while ensuring plastic formability of a steel sheet, in the case of obtaining a formed member of a steel sheet member having high strength by performing a nitriding treatment after plastic forming using a steel sheet material.
Also, it is an object of the invention to securely obtain a formed member having a part of sufficiently high strength in comparison with that of the other parts, concurrently to improve yield of the steel sheet material.
Furthermore, it is an object of the invention to allow the foam material to be fixed to the formed member with a relatively simple configuration, and to obtain a high reinforcing effect by filling of the foam, in reinforcing the formed member by filling the foam.
Furthermore, it is an object of the invention to obtain reliably a formed member of sufficiently high strength and to make it possible to perform a partial strengthening reliably and readily.
As described above, when reduction of the body weight and reduction of the production cost are intended by strengthening the steel sheet itself, in general, the effect (particularly economical effect) differs depending on the achieved strength of the steel sheet. In view of the above-mentioned technical problems, as the result of enthusiastic researches, the inventors of the present invention have found that the achieved economical effect significantly differs at a specific tensile strength (about 1,000 MPa) as the borderline, as shown in FIG. 5.
That is, when the tensile strength of the steel sheet is less than about 1,000 MPa, only the effect that the sheet thickness of the steel sheet of the body can be set thinner in accordance with the increase of the tensile strength, however, when the tensile strength of the steel sheet is about 1,000 MPa or more, not only capable of reducing the sheet thickness but also eliminating the necessity of a reinforcing member, so that not only the body weight and the material cost can be reduced, but also the mold cost and assembling process can be eliminated, which leads a significant economical effect.
When considering reduction of body weight by strengthening the steel sheet itself (that is, by reducing the thickness of the steel sheet material), in general, it is not preferred to select such body parts and structural members that require ensuring of not only the strength but also the rigidity from the view point of the body structure (in other words, attribution with respect to the rigidity of the whole body is high) as a subject to be made thinner for reducing the weight, because even if the strength is ensured by strengthening the steel sheet itself, the reduced sheet thickness adversely affects from the view point of ensuring the rigidity. However, it is preferred to consider to apply this measurement to such body parts and structural members in which attribution with respect to the rigidity of the whole body is relatively low, and requires to keep a certain strength or higher.
Furthermore, the inventors of the present invention found in the course of carrying forward the research and development for application of this nitriding treatment technique to a steel sheet of a vehicle body, that hardness of a steel sheet having been subjected to a nitriding treatment distributes in various ways in the sheet thickness direction in accordance with a difference in the steel sheet material itself and a difference in the treatment condition, however, there is a high correlation between the average hardness and the tensile strength, so that by setting the average hardness in the sheet thickness direction at a predetermined value, it is possible to obtain a desired tensile strength in correspondence with the set value. Moreover, for the same average hardness, the larger the difference in hardness between the surface part (surface and the vicinity) and the inside center part of the sheet material increases, the lower the tensile strength and the elongation property are, and in particular, if the difference in hardness exceeds a predetermined value, the tensile strength and the elongation property abruptly decrease.
Furthermore, as the result of enthusiastic researches in applying the tailored blank method to the production of bodies of automobiles, the inventors of the present invention have found that, as for the mechanical properties of the blank materials to be joined together by welding and the like, in the case where the difference in elongation property before press forming between the both blank materials is restrained to a certain small range or approximately same level, even if a plastic forming is conducted after welding, defects such as crack or break may not occur easily at the jointed portion.
Furthermore, the inventors of the present invention found that, by subjecting a steel sheet material to a heat treatment (nitriding treatment, in particular), an intermetallic compound (nitride, in the case of nitriding treatment) having microscopic porosity (so-called porous) in the surface and the vicinity of the steel sheet is generated, so that it is possible to obtain much higher adhesive property than the case of the steel sheet not having experienced a heat treatment.
Furthermore, as the result of enthusiastic researches in applying the hydroform process to the production of a body member of automobiles, the inventors of the present invention have found that, after forming a metal material containing a predetermined heat treating elements by the hydroform process, and then subjecting the formed member to a predetermined heat treatment, thereby, the hydroform process can be performed without trouble, and concurrently, it is possible to obtain a formed member of sufficiently high strength and to make it possible to perform a partial strengthening readily.
In view of the above, a method for producing a formed member made of a steel sheet member according to a first aspect of the present invention comprises the steps of: preparing a steel sheet material having tensile strength of 500 MPa or less and containing a nitriding element; forming a formed member having a predetermined shape by performing a plastic forming on the steel sheet material; and performing a nitriding treatment on the formed member so that an average hardness in the sheet thickness direction of the resultant steel sheet member is Hv 300 or more by Vickers hardness.
The upper limit of the tensile strength of the steel sheet member was set to 500 MPa because if the tensile strength exceeds this value, plastic formability for such as press forming is significantly decreases, so that it is difficult to conduct a plastic forming. It is to be noted that a commercially available so-called 440 MPa steel sheet, of which lower limit of the tensile strength in specification is 440 MPa, has actually a tensile strength in the range of about 440 MPa to about 500 MPa. Furthermore, the lower limit of the average hardness in the sheet thickness direction of the steel sheet material after the nitriding treatment was set to Hv 300, because as described above, in order to obtain a significantly large economical effect by making it possible not only to reduce the sheet thickness but also to eliminate the necessity of a reinforcing member, as well as by making it possible not only to reduce the body weight and the material cost but also to eliminate the necessity of mold cost and assembly process for the member, it is necessary that the tensile strength of the steel sheet is about 1,000 MPa or more, and it is necessary that the average hardness is Hv 300 or more for achieving this tensile strength.
According to the method for producing a formed member made of a steel sheet member according to the first aspect of the present invention, since the tensile strength of the steel sheet material is 500 MP or less, it is possible to sufficiently ensure the plastic formability for press forming and the like before nitriding treatment. Furthermore, since the average hardness in the sheet thickness direction of the steel sheet member is 300 Hv or more, the tensile strength of the steel sheet material is about 1,000 MPa or more, it is possible to obtain a significant large economical effect. For example, when applied to a panel member or a structural member for a body of an automobile, not only capable of reducing the sheet thickness but also eliminating the necessity of a reinforcing member, so that not only the body weight and the material cost can be reduced, but also the mold cost and assembling process can be eliminated, which leads a significant economical effect.
In a second aspect of the invention, the method for producing a formed member made of a steel sheet according to the first aspect is characterized in that the steel sheet material contains as the nitriding element, a predetermined amount of at least one element of titanium (Ti), niobium (Nb), boron (B), vanadium (V) and aluminum (Al).
It is preferred that the content of titanium (Ti) is 0.14 wt % or more, the content of niobium (Nb) is 0.1 wt % or less, the content of boron (B) is 0.005 wt % or less, the content of the vanadium (V) is 0.1 wt % or less and the content of aluminum (Al) is 0.06 wt % or less.
According to the second aspect of the present invention, basically, the same effect as the first aspect can be achieved. In particular, since the above steel sheet member contains a predetermined amount of at least one element of titanium (Ti), niobium (Nb), boron (B), vanadium (V) and aluminum (Al) as the nitriding element, it is possible to obtain the heat treating effect by the nitriding treatment with reliability.
Furthermore, according to a third aspect of the present invention, the method for producing a formed member made of a steel sheet according to the first aspect is characterized in that the difference in hardness between the surface part and the inside center part in the thickness direction of the steel sheet member of the formed member is Hv 200 or less by Vickers hardness.
The upper limit in the difference in hardness between the surface part and the inside center part in the sheet thickness of the steel sheet member of the formed member was set to Hv 200, because as described above, for the same average hardness in the sheet thickness direction, the larger the difference in hardness between the surface part (surface and the vicinity) and the inside center part of the sheet material increases, the lower the tensile strength and the elongation property are, and in particular, if the difference in hardness exceeds Hv 200, the tensile strength and the elongation property abruptly decrease.
According to the third aspect of the present invention, basically, the same effect as the first aspect can be achieved. In particular, even for the same average hardness in the sheet thickness direction, as the difference in hardness between the surface part (surface and the vicinity) and the inside center part of the steel sheet member increases, the tensile strength and the elongation property decrease, however, by setting the difference in hardness between the surface part and the inside center part in the sheet thickness direction of the steel sheet member of the formed member Hv 200 or less, it is possible to prevent the tensile strength and the elongation property from abruptly decreasing, and hence realize more stable strengthening.
Furthermore, according a fourth aspect of the present invention, the method for producing a formed member made of a steel sheet according to the first aspect is characterized in that only a specific region of the formed member is strengthened by the nitriding treatment, and when the formed member bent-deforms, it deforms at a boundary between the specific region and the unspecific region as an origin.
According to the fourth aspect of the present invention, basically, the same effect as the first aspect can be achieved. In particular, since only the specific region of the formed member is strengthened by the nitriding treatment, so that when the formed member is bent to deform, it deforms at the boundary between the specific region and the unspecific region as the origin of deformation, for a one-piece formed member, it is possible to desirably control the deformation mode at the time of bent deformation without necessity of providing another member such as joint member, partial reinforcing member and the like.
Furthermore, according to a fifth aspect of the present invention, the method for producing a formed member made of a steel sheet according to the fourth aspect is characterized in that before the nitriding treatment, a masking treatment is performed on the part other than the specific region of the formed member.
According to the fifth aspect of the present invention, basically, the same effect as the fourth aspect can be achieved. In particular, since the masking treatment is performed on the part other than the specific region of the formed member before the nitriding treatment, it is possible to strengthen only the specific region of the formed member with reliability and readiness by the nitriding treatment.
Furthermore, according to a sixth aspect of the present invention, the method for producing a formed member made of a steel sheet according to the fourth aspect is characterized in that the nitriding treatment is performed while only the specific region of the formed member is immersed in a salt bath.
According to the sixth aspect of the present invention, basically, the same effect as the fourth aspect can be achieved. In particular, since the nitriding treatment is performed while only the specific region of the formed member is immersed in the salt bath, it is possible to reliably strengthen only the specific region of the formed member by the nitriding treatment without necessity of providing another process.
Furthermore, according to a seventh aspect of the present invention, the method for producing a formed member made of a steel sheet according to the first aspect is characterized in that forming the formed member having a predetermined shape is performed by a method comprising the steps of: preparing a first blank material and a second blank material having different properties for a predetermined specification as the steel sheet materials; forming a preform by joining those blank materials; and performing a plastic forming on the preform to obtain a formed member of a predetermined shape.
According to the seventh aspect of the present invention, basically, the same effect as the first aspect can be achieved. In particular, since after plastic forming the preform which has been formed by joining the first blank material and the second blank material having different properties for a predetermined specification, a specific part of the formed member obtained by the plastic forming is hardened by the nitriding treatment, it is possible to sufficiently ensure the plastic formability before heat treatment. And by the nitriding treatment performed after plastic forming provides the specific part of the formed member with a desired strength. That is, by applying the tailored blank method, it is possible to obtain a formed member having a part therein of which strength is sufficiently higher than other parts, while improving yield of the sheet-like material.
Furthermore, according to a eighth aspect of the present invention, the method for producing a formed member made of a steel sheet according to the first aspect is characterized in that the formed member has a closed section shape and is reinforced by a method comprising the steps of: setting a foam material to a region subjected to the nitriding treatment; and causing the foam material to expand by heating the formed member.
According to the eighth aspect of the present invention, basically, the same effect as the first aspect can be achieved. In particular, since after forming a formed member having a closed section shape by subjecting a sheet material containing the nitriding element to a plastic forming, the nitriding treatment is performed on at least a predetermined region of the formed member, an intermetallic compound having porosity (so-called porous) in the surface and the vicinity of the surface with respect to this predetermined region is generated. Then, since after setting a foam material to this predetermined region, the foam material is caused to expand by heating the formed member, the foam material is expanded and hardened with respect to the predetermined region of which surface and the vicinity are porous, so that it is possible to obtain much higher adhesive property than the case of the steel sheet not having experienced a heat treatment. That is, in reinforcing the formed member by filling the foam, it is possible to allow the foam to be fixed to the formed member with a relatively simple configuration, and to obtain a high reinforcing effect by filling of the foam.
Furthermore, according to a ninth aspect of the present invention, the method for producing a formed member made of a steel sheet according to the first aspect is characterized in that forming the formed member having a predetermined shape is performed by a method comprising the steps of: forming a preform having a closed section shape which is relatively approximate to the final shape of the formed member; setting the preform in a predetermine mold; and forming the preform by supplying the closed section space thereof with a pressurized fluid to obtain the formed member corresponding to a shape of the mold.
According to the ninth aspect of the present invention, basically, the same effect as the first aspect can be achieved. In particular, since the preform having a closed section shape which is relatively approximate to the final shape of the formed member is located in the predetermine mold, and by supplying the closed section space of the preform with a pressurized fluid, the formed member is hardened by the nitriding treatment after forming the formed member corresponding to the shape of the mold, it is possible to ensure satisfactory plastic formability at the time of forming before the heat treatment. And then, by the nitriding treatment conducted after the plastic forming, it is possible to provide the specific region of the formed member with a desired strength. That is, in obtaining a formed member by employing the so-called hydroform process, it is possible to reliably obtain the formed member having satisfactory strength.
Furthermore, a formed member made of a steel sheet member according to a tenth aspect of the present invention has an average hardness in the sheet thickness direction of Hv 300 or more by Vickers hardness by plastically forming a steel sheet member having a tensile strength of 500 MPa or less and containing a nitriding element into a predetermined shape and performing a nitriding treatment after the plastic forming.
The upper limit of the tensile strength of the steel sheet member was set to 500 MPa or less and the lower limit of the average hardness in the thickness direction of the steel sheet member after the nitriding treatment was set to Hv 300 for the same reason as is the case of the first aspect of the invention.
According to the formed member made of a steel sheet member according to the tenth aspect of the present invention, since the tensile strength of the steel sheet material is 500 MPa or less, it is possible to sufficiently ensure the plastic formability such as press forming before the nitriding treatment. Furthermore, since the average hardness in the sheet thickness direction of the steel sheet member after the nitriding treatment is Hv 300 or more, it is possible to make the tensile strength of this steel sheet member 1,000 MPa or more, so that significantly large economical effect can be obtained. For example, when applied to a panel member or a structural member for a body of an automobile, not only capable of reducing the sheet thickness but also eliminating the necessity of a reinforcing member, so that not only the body weight and the material cost can be reduced, but also the mold cost and assembling process can be eliminated, which leads a significant economical effect.
Furthermore, according to a eleventh aspect of the present invention, the formed member made of a steel sheet member according to the tenth aspect is characterized in that the formed member is formed to a predetermined shape by a plastic forming on a preform obtained by joining a first blank material and a second blank material having different properties for a predetermined specification.
According to the eleventh aspect of the present invention, basically, the same effect as the tenth aspect can be achieved. In particular, since after plastic forming the preform which has been formed by joining the first blank material and the second blank material having different properties for a predetermined specification, a specific part of the formed member obtained by the plastic forming is hardened by the nitriding treatment, it is possible to sufficiently ensure the plastic formability before heat treatment. And by the nitriding treatment performed after plastic forming provides the specific part of the formed member with a desired strength. That is, by applying the tailored blank method, it is possible to obtain a formed member having a part therein of which strength is sufficiently higher than other parts, while improving yield of the sheet-like material.
Furthermore, according to a twelfth aspect of the present invention, the formed member made of a steel sheet member according to the tenth aspect is characterized in that the formed member has a closed section shape and at least a part of the formed member is filled with foamed material, by setting a foam material to at least a region subjected to the nitriding treatment and causing the foam material to expand by heating the formed member.
According to the twelfth aspect of the present invention, basically, the same effect as the tenth aspect can be achieved. In particular, since after forming a formed member having a closed section shape by subjecting a sheet material containing the nitriding element to a plastic forming, the nitriding treatment is performed on at least a predetermined region of the formed member, an intermetallic compound having porosity (so-called porous) in the surface and the vicinity of the surface with respect to this predetermined region is generated. Then, since after setting a foam material to this predetermined region, the foam material is caused to expand by heating the formed member, the foam material is expanded and hardened with respect to the predetermined region of which surface and the vicinity are porous, so that it is possible to obtain much higher adhesive property than the case of the steel sheet not having experienced a heat treatment. That is, in reinforcing the formed member by filling the foam, it is possible to allow the foam to be fixed to the formed member with a relatively simple configuration, and to obtain a high reinforcing effect by filling of the foam.
Furthermore, according to a thirteenth aspect of the present invention, the formed member made of a steel sheet member according to the tenth aspect is characterized in that the formed member is formed to a predetermined shape by forming a preform having a closed section shape which is relatively approximate to the final shape of the formed member, setting the preform in a predetermine mold, and forming the preform by supplying the closed section space thereof with a pressurized fluid to obtain the formed member corresponding to a shape of the mold.
According to the thirteenth aspect of the present invention, basically, the same effect as the tenth aspect can be achieved. In particular, since the preform having a closed section shape which is relatively approximate to the final shape of the formed member is located in the predetermine mold, and by supplying the closed section space of the preform with a pressurized fluid, the formed member is hardened by the nitriding treatment after forming the formed member corresponding to the shape of the mold, it is possible to ensure satisfactory plastic formability at the time of forming before the heat treatment. And then, by the nitriding treatment conducted after the plastic forming, it is possible to provide the specific region of the formed member with a desired strength. That is, in obtaining a formed member by employing the so-called hydroform process, it is possible to reliably obtain the formed member having satisfactory strength.